Like why in general.. and the benefits and such which the WIkipedia is seeing.
PHP 4 is in maintenance mode. PHP 5 is being actively developed, has many new features, and is faster in many cases. It is the latest, greatest version. IMO, there is no legitimate reason to continue developing PHP 4 code.
The only other discussion I've ever heard with regards to PHP5 was from other developers talking about how much of a letdown it was, and how much of a moving target it has been from minor revision to minor revision, the breach of promises of backwards compatability, the introduction of new oddities and not solving previous oddities etc. All negative stuff. So I'm curious to see the mediawiki team perspective.
FUD, FUD, FUD, and more FUD. PHP devs went to great ends to make PHP 5 very compatible with PHP 4. Most PHP 4 scripts ran just fine under PHP 5 without any modifications. The scripts that needed modifications needed all but a few single-liners, which could easily be found via grep. 99% of scripts that needed a heavy-lifting to work with PHP 5 were not written well to begin with. If you want to talk about PHP upgrades breaking things, you should read the developer notes about PHP 6. They plan to turn off a lot of the support for legacy PHP 4. It is kind of scary, actually. I guess it's the price one pays when a language evolves.
To answer your question, I don't recall a discussion about going from PHP 4 to 5. The devs kinda just decided it was a good idea, and rightfully so. I saw a Subversion commit with an exception one day, and the rest is history.
The only concern I have with the decision is that doing a PHP 4 to 5 transition is a large overtaking. By the time you have gotten rid of all the PHP 4 classes, you might as well have done a full rewrite. There is just so much auditing that needs to be done once you put protected/private in front of properties and methods that it might be worth spending the time doing a rewrite.
Greg
BTW, I am not affiliated with Wikimedia, but am an avid MediaWiki hacker. My PHP coding has also been limited to version 5 since August of 2004. I only touch version 4 when forced.